"CSS" stands for Cascading Style Sheets in web design. It is a stylesheet language used to describe the presentation and layout of web pages written in HTML or XML. CSS allows designers to apply styles such as colors, fonts, spacing, and positioning to elements on a webpage, significantly enhancing its aesthetic appeal and user experience. With CSS, developers can create responsive designs that adapt to different screen sizes and devices, making it a fundamental tool in modern web development for achieving attractive and effective user interfaces.